Charged particle beam systems, e.g. scanning electron microscopes (SEMs), electron beam probers and focused ion beam (FIB) systems, are used in a variety of ways to aid in the diagnosis and repair of IC devices. For example, FIB systems are used to mill away material overlying a conductor so as to expose the conductor for modification such as severing or connection with other conductors, or repair. When a conductor of interest is hidden by overlying layers, it is necessary to provide some manner in which it can be located to allow accurate positioning for milling. There have been various previous proposals for dealing with this problem.
Traditionally, structures within IC devices (dies) have been accessible by charged particle beam devices from the front surface of the device. However, when the region of interest lies below a power plane, the structures cannot be seen by any method and even when the power plane is removed, there are no topological features which can be seen in a FIB or e-beam image. While it is possible to see through the substrate, silicon being transparent at infra red wavelengths, or when the power plane is removed, it is extremely difficult to engineer a system in which an optical system can have the same field of view as a FIB system. It is also difficult and expensive to produce a moveable stage to allow visual imaging to be used to locate a FIB site on a die at one location which is then moved to another for FIB processing.
U.S. Pat. No. 4,683,378 describes four different approaches for controlling a FIB milling operation in which structures of interest are hidden by overlying layers. The first approach is to compare scanning ion microscope image with CAD data to infer the position of a site of interest. The second approach is to position a reference die under an optical microscope and use the optical image of the reference die to located the corresponding position in the SIM image. The third approach is to move the die between an optical microscope and the SIM and compare the images from both to find the location of interest. The fourth approach is to move the die between optical, electron beam and scanning ion microscopes to provide images with different objects visible or different resolutions. These last three approaches requires the use of a high accuracy stage to move the die between the microscopes for registration of the images. Other proposals for using optical images in FIB processing generally fall into one or other of these approaches.
It has also been proposed to attempt to overcome the problem by using a FIB to place a mark on a die, moving the die to an optical microscope and locating the mark so as to determine its position in relation to the structures in the die, and then moving the die back to the FIB and using the mark and the optical information to navigate during a FIB milling operation. This approach has the problems that FIB marking takes a relatively long time, three steps are required and two movements of the die are needed to move between the optical and FIB locations.
It is an object of the present invention to provide a system which allows optical techniques to be combined with charged particle techniques for processing dies without the problems identified above.